The present invention relates to a screw compressor suitable for use in producing high-pressure air or high-pressure gas and for raising the pressure of a refrigerant in a gaseous state.
A typical screw compressor is shown in, for example, U.S. Pat. No. 3,423,017, wherein, a pair of rotors are each formed with a plurality of spiral lands or lobes, and a plurality of grooves are defined between the lobes which constitute compression chambers. An outer peripheral portion and end portions of the two rotors are positioned close to an inner wall surface of a rotor casing with a small gap existing therebetween except at a location of a gas suction port and a gas discharge port, so that the two rotors are substantially enclosed by the rotor casing. Due to this construction, the grooves formed on the rotors provide closed spaces other than suction and discharge strokes, and each space undergoes a change in volume (reduction) as the rotors rotate to enable the gas to be drawn by suction, compressed and discharged from the compression chamber.
In the aforementioned compressor of the type, gaps are defined between the two rotors and between the two rotors and rotor casing. In this construction, the gas of high temperature leaks from the grooves on the higher pressure side to the grooves on the lower pressure side. Additionally, the gas of high temperature leaks into the groove in suction stroke and occupies a part of the groove, thereby reducing the suction efficiency of the compressor.
Presuming, for example, that the commpressor is a single-stage, oilless air compressor of a discharge pressure of 7 kg/cm.sup.2, discharged air has a temperature of more than 300.degree. C., and when the air leaks through the gaps, the volume of the amount of the air leaked into the groove in suction stroke would be about twice as much in comparison with a volume of the same amount of air at suction temperature, because the air shows almost no change in temperature before and after passing through the gap. Thus, the weight of air in the groove at completion of the suction stroke becomes smaller as compared with a case in which leakage is not occurred. The work of compressing air from the suction state to the discharge pressure level has almost nothing to do with the temperature of air when the volumes of air to be compressed are the same. However, the higher the temperature of air at the completion of the suction stroke, the lower the weight of air becomes even if the volumes are the same. This means that the work per unit air weight increases and decreases efficiency of the compressor.
The above description has been provided in connection with an air compressor by way of example; however, it will be appreciated that this is not an isolated phenomenon and that the same problem would be encountered when a gas other than air is compressed.